1. Field of the Invention:
The present invention relates generally to optical switches, and more particularly to a position controller for an optical switch. Still more particularly, the present invention relates to a precision micro-motor position controller.,
2. Description of the Prior Art:
Efficient and economic storage of digital information is an important consideration of manufacturers, designers and users of computing systems. In optically readable storage devices, digital data is typically stored in tracks. located on rotating disks of optically readable storage media. Close positioning of the adjacent disk tracks maximizes the amount of stored data on a storage disk, thus providing significant economic benefits to system manufacturers and users.
One technique that can be used to position adjacent disk tracks closely is optical recording. Optical recording requires an optical fiber to direct light to and from the optical disk. When there is more than one flying optical head in the optical disk drive system, the light from a common optics module must be switched between optical fibers. The light is switched by positioning a focused light beam on the ends of one fiber that contains several fibers therein. A precision micro-motor and accurate position sensing of the micro-motor is needed to implement the switching control function.
FIG. 1 is a diagram of an optically readable storage system that can utilize the precision micro-motor position controller of the present invention. Optically readable storage system 100 includes an optics assembly 102 coupled to a fiber optics switch 104 by an optical fiber 106. Optics assembly 102 is also coupled to a drive module 108. via signal lines 110, 112. A servo module 114 is coupled to the drive module 108 via line 116. Storage system 100 further includes an actuator 118, a plurality of optical fibers 120, a plurality of head arms 122, a plurality of suspensions 124, a plurality of heads 126, and a plurality of optically readable storage media 128.
Each of the plurality of optically readable storage media 128 is typically mounted on a spindle 130 for continuous rotation at a constant angular velocity. Each of the plurality of heads 126 is attached to the actuator 118 by a respective flexible suspension 124 and head arm 122.
To read data from an optically readable storage media 128, light is reflected from the optically readable storage media 128 back through the head 126 to one of the plurality of optical fibers 120. Typically, the plurality of optical fibers 120 are contained in one fiber bundle. In order to find the correct optical fiber, a raster scan of the bundle is performed using a x position device and a y position device. When each optical fiber is located, the position is noted so a fine scan can be performed for each fiber independently.
FIG. 2(a) is a top view of a portion of a prior art position sensing circuit. This prior art position sensing circuit is known in the industry as a lenslet array switch 200. A plurality of fibers 202, are positioned in a straight line and a plurality of lens are molded as one unit in a corresponding straight line. The straight line of lens is known as a lenslet array 204.
A collimating beam 206 is guided to a desired lenslet 204 by the rotating mirror 208. The rotating mirror 208 is positioned by a micro-motor. A redirection lens 210 is used to perform fine adjustments of the collimated beam 206 through the lenslet 204 onto a fiber. The redirection lens 210 can also perform high speed dithering of the focused light beam on the fiber to ensure the best optical coupling is maintained.
Referring to FIG. 2(b), a position sensing device (PSD) 212 provides position feedback to the rotating mirror 208. To generate the position feedback, a 5% beam splitter 214 splits off a portion of the collimated beam 206 to a 50/50 beam splitter 216, where 50% of the beam is guided to the PSD 212. The other 50% of the collimated beam is directed towards the redirection lens 210 and then to a 2xn (two by n) array of photo detectors 218. The photo detectors 218 provide position sensing signals that control the redirection lens 210.
One of the disadvantages to this system is its complexity. Manufacturing the components of the system is an involved process, and constructing the system with the components is complicated. This is particularly true for the position sensing devices, the photo detector array, and the beam splitters. Because of its complexity, creating the system is an expensive process. This is another disadvantage to contemporary position sensing systems.
Another type of micro-motor control system is disclosed in a paper entitled xe2x80x9cThe Interface and System Considerations of Microactuators for Magnetic Disk Drivexe2x80x9d by L.-S. Fan, J. Hong, T. S. Pan, S. Pattanaik, T. Hirano, T. Semba and S. Chan (IEEE Proc. of Transducers ""99, pp. 1054-1057, June 1999). The control system drives the armature with a driver and measures the voltage difference between a resistor and capacitor and the armature. The system does not require high precision positioning, and is accurate to only one part in four hundred.
For an optical disk system, however, this accuracy is not acceptable. Optical disk drives utilize long arrays of fibers that require a focused beam to accurately find a particular fiber and then maintain optical coupling with the fiber. Additionally, error measurement is one of the more critical aspects of a micro-motor position controller. To operate at an optimal level, the system must be able to make very precise error measurement. Thus, a need exists for an improved micro-motor position controller.
The present invention overcomes the limitations of the prior art systems by providing a micro-motor position controller that determines a position error signal by a differential voltage measurement relative to ground. In the exemplary embodiment, a micro-motor is comprised of a first stator, a second stator, and an armature. A first capacitor is created between the first stator and the armature. A second capacitor is created between the second stator and the armature. To move the armature, a higher voltage difference is produced between one stator and the armature while a smaller voltage difference is created between the other stator and the armature. The first and second capacitors are connected in series in order to form one section of a capacitive bridge.
In one exemplary embodiment two multiplying digital-to-analog converters (DAC) are used to provide sinusoidal voltages to the top and bottom locations of the one section of the capacitive bridge. The other section of the capacitive bridge is ground (no sinusoidal voltage). The sinusoidal voltage at the top of the bridge and the sinusoidal voltage at the bottom of the bridge are out of phase by 180 degrees. The motor is moved and the capacitance values of the first and second capacitors are changed until no voltage is present on the armature. This allows the voltage level at the armature to be measured relative to AC ground and then used as a feedback signal to control the motor. The voltage values from the multiplying DACs determine the set point for the motor position. In the exemplary embodiment, the gains of the DACs are adjusted in a complementary fashion.
In an alternative exemplary embodiment, a micro-motor position controller is comprised of a high voltage amplifier circuit, a bridge, a position set/bridge driver circuit, and a low capacitance preamplifier/overload protection circuit. A digital potentiometer and a bridge driver are used to set the ratio of the capacitance values of the first and second capacitors of the micro-motor. The low capacitance preamplifier senses an error signal for the micro-motor. The error signal drives the high voltage amplifier so that the ratio of the capacitance values of the first and second capacitors mimics the ratio of the input signals created by the bridge driver and the digital potentiometer.
Another aspect of the alternative exemplary embodiment includes the use of a balanced modulator. The balanced modulator is implemented with two signal channels. One channel is comprised of the signal plus some error (signal+error). The second channel is comprised of (xe2x88x92signal+error). Subtracting the two channels generates the output of balanced modulator. This causes the error to be subtracted out of the output signal. This is highly desirable, because any error in the output is equivalent to an error in the set point of the motor.